This invention relates to a speed change device provided on a crankshaft of a bicycle.
A widely used speed change device for a bicycle includes multistage sprockets and an endless chain which is trained about one set of sprockets and is transferred to other sets of sprockets by means of a delayer for the purpose of changing speeds. This is of a type of a speed change device provided externally of a bicycle. This type of the speed change device has disadvantages in that the chain is likely to disengage from the sprockets in running and does not smoothly transfer from one set of sprockets to the other so that a smooth speed change is difficult. Moreover, this device produces noise when speed changing, and exposed sprockets outwardly extending are likely to touch trousers of a cyclist.
There is another type of a speed change device built in a hub of a rear wheel, which is not used so much. This mechanism is sophisticated and apt to cause trouble.
A speed change device including a planetary gear mechanism cooperating with a crankshaft has been disclosed in Japanese Utility Model Application Publication No. 19/53. This type of the speed change device is superior to those above described. However, this speed change device disclosed the Japanese Publication No. 19/53 is complicated in construction and has a limitation in that speed change more than three steps is impossible from the arrangement of the planetary gear. It is therefore difficult to realize the idea disclosed in the Japanese Publication as a trade product.